Piston



Patented May 6, 1941 William M. Venner, Richmond Heights, and Percy L.Bowser, Jr., University City, Mo., assignors t The Sterling Corporation,St. Louis, Mo., a corporation of Delaware Application October 12, 1939,Serial No. 299,050 11 Claims. (Cl. 3 09-13) This invention pertains topistons particularly for internal combustion engines, and moreparticularly to pistons of the trunk type made of light metal, such asaluminum or magnesium or alloys thereof.

. A major problem in the construction of all light metal pistons hasbeen to provide accurate compensation for the high rate of thermalexpansion of the piston metal, particularly aluminum and its alloys.This, of course, is due to the fact that the rate of thermal expansion,which is measured bythe coefcient of expansion, is about twice as greatfor aluminum as it is for cast iron, and, since an aluminum piston mustordinarily work within a cast iron cylinder, this excess expansion ofthe piston would require an excessive clearance between piston andcylinder if no compensation for expansion could be made.

The amount of compensation required is `somewhat diierent at the Vupper.or head end of the piston than it is at the lower end. This is chieflyon account of the fact that there is a fallingtemperature gradient fromthe head end to the lower end since the piston head is heated ,by thecombustion gases, and this heat ows downwardly through the piston to thecylinder walls where it is delivered to the cooling water surroundingthose walls.

In the all-aluminum commercial pistons now skirt in the regionsvof thecylinder-bearing portions or thrust'faces, and the skirt is relieved inthe regions of the pin bosses or bearings, while the head diameter isreduced. The piston is so constructed that the thrust faces will fitwith the desired clearance when the piston is cold, As the piston heatsup, while the `head will expand uniformly, the head expansion-istransmitted to the skirt in the regions of the bosses, where there isrelief, and thethrust faces are drawn in. While this does, to an extent,compensate for expansion the thrust faces bear heavily at their ends orcorners, particularly at the top. This bearing is the result not only ofthe outward movement along the wrist-pin axis, but also cordal movementalong the connectors between vthe head bosses and skirt. Commercialpistons are therefore further relieved at these corners by what is knowna's a keystone relief, or by cam grinding of the skirt to an ellipticalform with the lminor axis of the ellipse along the wrist-pin axis. isobviated in a strut piston in which the cordal members are of a materialhaving a .rate of exin the direction along the pin axis. Strut pistonsare, therefore, also cam ground. However, whether keystone reliefs orcam grinding is em ployed, this will not only fail to secure a uniformbearing face when the piston is cold, but will also tend to cause sideslap.

4'Ihe purpose of this invention, generally stated,v is to provide apiston which can be constructed practically entirely of light metal andwhich has its structure so organized as 'to compensate to a certaindegree for the excessive rate of expansion r of the piston metal, withthe addition that near the upper part of the bearing portion of theskirt additional control of such expansion is introduced by adding oneor more control elements made of a material having a lower rate ofthermal expansion than the piston material. In accordance with anembodiment of this invention, such control element is so arranged andpositioned as to control the curvature of the upper part of the bearingportion of the skirt.

The further object of this invention is to provide a piston structure socontrolled as to its expansion that the bearing portion may be finishedto substantially a true circular shape and still maintain a good bearingfit in the cylinder at all operating temperatures.

Further`- objects will appear from the following description, taken inconnection with the accomv panying drawing, in which:

Figure 1 is a side view taken along the wrist pinv axis of a pistonembodyingthis invention;

Figure 2 is a side view of the piston of Figure 1 showing anotherembodiment; V

Figure 3 is a longitudinal section taken in two planes, indicated by thelines 3-3 of Figure 5; Figure 4 is a side view similar` to Figure 2 ofanother piston embodying this invention;

While the cordal expansion l Figure 5 is a transverse section taken intwo planes, indicated by lines 5--5 of Figure 3, and; Figure 6 is aperspective viewof one form of control element.

Generally statedand in accordance with this invention, a light metalpiston comprising a head,

. a skirt and wrist-pin bearings connected with the connectors betweenthe head and the skirt. An

element having a rate of yexpansion lower than 'the cylinder-engagingportion is arranged to ex\ the cylinder-engaging portion, as by casting,so as toflex the cylinder-engaging portion inwardly in the regions ofthe vbearings thermostatically.

Referring now to the drawing, the piston shown is cast substantiallyentirely of aluminum alloy and has a head I provided with the usualpacking ring grooves 2. Wrist pin bearings 3 are secured to the head Iby downwardly extending connectors 4, reinforced by ribs 5, The pistonskirt,

indicated generally at 6, is attached to the head and between thecylinder-engaging portions 1, are relieved, as indicated at 8. Thisrelief is provided preferably by slightly indenting this portion of thecasting so that its maximum diameter is sufliciently less than thecylinder bore to provide ample clearance at all temperatures.

In order to provide for proper operation of the piston in the cylinder,that portion of the head I which carries the packing rings in thegrooves 2 is machined to a relatively small diameter, providing suicientclearance so that this head portion will never contact with the cylinderwalls chined to a small enough diameter to provide ample clearance atall times. The cylinderengaging portions 1, however, are finished tosuch a diameter as to allow just sufficient clearance to provide asmooth sliding iit under proper conditions of lubrication. Theseportions function to guide the piston and 'support the lateral thrusts.due to angularity of the connecting rod. In accordance with theembodimentshown in Figures 1, 2, and 3, the cylinder-engaging portions 1are separated from the head I by-horizontal slots or gaps 9 and from thebearings 3 and connectors 4 by downwardly extending slots I0. In theparticular embodiment illustrated, the slots I0 extend downwardly andturn laterally, as indicated at I I, so as to form at the upper lateralcorners of the cylinder-engaging portions 1 laterally extending wingsI2. The limiting edge of the relieved area 8 may beextended upwardly atan angle, as indicated at I3, so as to carry this relieved area aroundthe ends of-the lateral slots II.

The relieved area 8 may be carried clear to the lower edge of the skirton both sides, as shown in Figure 1. 'It may, however, be desirable forsome purposes to limit the downward extent of this area and provide atthe lower end of the piston a circumferential rim portion I4, arrangedto bear against a cylinder wall. Such a rim por-l tion may be providedat only one side of the piston, as shown in full lines in Figure 2,v orsuch a portion may be provided on both sides, as indicated in dottedlines in that ligure. 'I'hese variationsvare determined in mostinstances by the vrequirements of particular engines.

In accordance with the illustrated embodiments of this invention, acontrol element I5 is provided having a lower rate of thermal expansionthan the piston metal. This element may be constructed of steel, whichis cheap and easily worked, and having a coefiicient of expansion lessthan that of the piston metal, or it may beof other suitable materialhaving thefproperl c0- eicient of thermal expansion. In the embodimentillustrated, it is provided in the form of a flat strip of arcuate shapeand provided `on-its outer circumference with a series of recesses I6,adapted to interlock withthe piston metal during casting. In theembodiment shown, the recesses I6 are formed by simply drilling orpunching round holes close enough to the outer rim of the element I5 soas to overlap said rim, leaving the recesses open at the outside.

One or more elements I5 are applied to the piston by being supported inthe mold iny any v suitable manner when the piston is cast, so as to becast into the piston meta-l in the position shown in Figures 1 to 5,inclusive. As will be seen from Figure 5, the element I5 extendscircumferentially around the piston skirt along the inside thereof, andthe piston metal flows into the recesses I6 during the casting operationso as to provide a secure interlock between th element I5 and the pistonskirt. It will be seen that the element is positioned so as to extendhorizontally along the inside of the cylinderengaging portion of theskirt near its top and soas to extend circumferentially in bothdirections from the middle of the `portion 1 toward the outer topcorners and the wings I2.

It will be noted, particularly from Figure 5,

I that the element 'I5 is so related to the cylindereven at the highestoperating temperatures. f Similarly, the ends of the bearings 3 aremaengaging portion 'I as to provide therewith a bimetallic memberextending horizontally along the top part of said portion.' Such avbi-metallic member may act thermostatically upon increase oftemperature, and, since the less expansible member I5 isv on the insideof `the curve, rise of temperature will tend to cause an inward curlingof thisbi-metallic part so as to tend to draw radially inwardly thelateral top corners of the l end. It has been found that the controlelement I5 may be so proportioned relatively to the `piston skirt thatthis thermostatic action may be adjusted to compensate almost exactlyfor the excess expansion of the aluminum piston over that of the ironcylinder.

It will be noted that the arrangement of the slot I0 is such as to freethe outer -top corners of the cylinder-engaging,portions 1 from thewrist pin bearings and thereby render said corners more radially.flexible. Accordingly, the full benefit of the thermostatic action justdescribed may be obtained at these upper corners.

At the -lower end of the skirt the excess ei:-v pansion of the pistonover that of the cylinder is less than at the top. Such excess may be'taken care of by providing the relief 8, which permits the expansion ofthe wrist pin bearings 3 as transmitted to the lower end of the skirt inthe direction along the pin axis, to stretch the lower end of the skirtin that direction and thereby tend to draw inwardly the lower ends ofthe portions 1.

Generally stated, the operation is as follows,

referring to the drawing: When the piston is,

cold the cylinder-engaging portions, particularly at the top, willengage the cylinder substantially circumferentially so that the pistonis supported in such a manner-that no slapping will take place. As thehead of the piston heats up, the head expansion is. of course,transmitted to the skirt along the wrist-pin axis, which will normallyresult in a drawing-in of the'cylinderengaging portions. The bimetallicstructure will, however, cause the terminals of the bearing faces at thetop to move in so as to maintain the engagement as before and so as toavoid rubbing at the corners. Accordingly, the skirt will maintain itsshape circumferentially as the piston heats up and will follow theexpansion of the cylinder.

It will be seen, therefore, that inaccordance with this invention, apiston is provided which is substantially entirely of cast light metal.The

excess expansion at the lower end of the'skirt is taken care of by thelight metal structure of the piston, while at the upper end of the skirtwhere the temperature is higher the control I operates to draw in theupper side corners of the cylinder-engaging portions so as to compensatefor excess expansion at this point. It has been found that with thisconstruction the portions 1 may be finished to substantially a truecircular shape instead of the eliptical shape, which has become almoststandard practice for aluminum pistons. When so finished to circularshape, this piston maintains a nearly perfect fit in the cylinder at alloperating temperatures. Pistons of this construction have. been fittedwith a clearance of .0015 to .002 inch when cold, and have givenperfectly satisfactory operation under all conditions of load. y

In the embodiment of Figure 4, the slots IM around to provide thehorizontal portions II. This provides a somewhat stiffer skirt at thetop, while still freeing the upper corners from the pin bearings or theconnectors l, so as to permit those corners to flex. In this embodiment,also, the side edge i3 of the relieved area 8 has been straightenedsomewhat. This provides a broader cylinder-engaging portion 1.

Such a structure may be desirable under conditions where a stiffer skirtis required. It is believed, also, that in this embodiment thecontrolling action ofthe member 'I5 may be carried downwardly along theskirt to a greater extent than in the embodiment'of Figures 1 and 2.

It will be understood, of course, that in any of the embodiments any ofthe arrangements of the relieved area 8, vshown in Figures l and 2, maybe employed, according to the requirements of the particular situation.Furthermore, the slots I0 and Il maybe extended more or less so as tocontrol the exibility of the upper corners of the bearing portion, or toprovide wings l2 of greater or less extent.

with said cylinder-engaging portion so as to extend circumferentiallyalong and within the upper part thereof toward said side corners, theupper part of said cylinder-engaging portion and said controlelement'forming a bi-metallic structure thermostatically controlling theexpansion of said side corners.

2. A light-metal piston of the character described, having a head,wrist-pin bearings rigidly connected with said head, a skirt connectedwith said bearings but having a cylinder-engaging portion separated fromsaid head inthe region between said bearings, and said cylinderbearingportion having upper side corners free of said pin bearings so as to beflexible relatively thereto, and a flat control element of a materialhaving a lower rate of thermal expansion than the piston metal, castinto and interlocked with said cylinder-engaging portion so as to extendcircumferentially along and within the upper part thereof toward saidside corners with its flat face in a plane transverse to the pistonaxis, the upper part of said cylinder-engaging portion and said controlelement forming a bi-metallic have been terminated without carrying theml It is obvious that various changes in the de-l t scope of the appendedclaims without departing from the spirit of this invention, and it isunderstood the invention is not limited to the specific details shownand described.

The invention having thus been what is claimed is:

l. A light-metal piston of the character described, having a head,wrist-pin bearings rigidly connected with said head, a skirt connectedwith said bearings but having a cylinderengaging portion separated fromVsaid headin the region between said bearings, said cylinderengagingportion having upper side corners free of said pin bearings so as to beflexible relatively thereto, and a control element of a material havinga lower rate of thermal expansion than the piston metal, cast into and`interlooked described,

with said bearings but having a cylinder-engag- `ing portion separatedfrom said head and terminating circumferentially in the regions between`said bearings to provide upper side corners, and a control element of amaterial having a'lower rate of thermal expansion than the piston metal,secured to the inside of said cylinder-engaging portion and extendingbetween the upper side corners thereof to control the curvature of saidportion thermostatically.

4. A light-metal piston of the character described, having a head,wrist-pin bearings rigidly connected with said head, a skirt connectedwith said bearings but having a cylinder-en- `gaging portion separatedfrom said head and terminating circumferentially in the region betweensaid bearings to provide upper side curves, and a control element of a`material having a lower rate of thermal expansion than the pistonmetal, secured to the inside of said cylinder-engaging portion andextending between the upper side corners thereof to control thecurvature of said bearing portion thermost-atically, said skirt beingfinished at its cylinder-engaging portion to a substantially truecircular form but being relieved in the region adjacent an end of thewrist-pin axis.

5,. A light-metal piston of the character described, having a head,wrist-pin bearings rigidly connected with said head, a skirt connectedwith said bearings but having a cylinder-engaging portion separated fromsaid head in the region between said bearings, said cylinder-engagingportion having 'a laterally extending wing scribed, having a head,wrist-pin bearings rigidly scribed, having a head, a skirt, wrist pinbearings connected with said head and said skirt, said skirt having acylinder-engaging portion whose upper side corners are separated fromsaid head and said bearings so as to be flexible relatively thereto, anda control element of a ma-f terial having a lower rate of thermalexpansion Vthan the piston metal, cast into and interlocked with saidcylinder-engaging portion so as to extend circumferentially along andwithin the upper part thereof toward said side corners, the upper partof said cylinder-engaging portion and said control element forming `abi-metallic structure thermostatically controlling the expansion of said'side corners.

bearings, connectors between said head and said skirt, said skirt havinga cylinder-engaging portion separated from said head circumferentiallyand separated from said connectors terminally,

8. A light metal piston of the character described, comprising, a head,a. skirt, wrist pin bearings connected with said head and said skirt,said skirt having a cylinder-engaging portion separated from said headand constructed to fiex inwardly in the regions of said bearings, andari-element having a rate of expansion. vlower than, extendingcircumferentially inside of and attached to, said cylinder-engagingportion, adapted to flex said portion inwardly in said regionsthermostatically.

9. A light metal piston of the character described, comprising, a head,a skirt, wrist pin constructed to iiexinwardly in the regions of saidbearings,` and an element having arate ofexpansion lower than said'cylinder-engaging portion, said element extending circumferentiallyinside of and cast to said portion and terminating therewith, adapted toex said portion inwardly in said regions thermostatically.

10. A light metal piston of the character described, comprising, a head,a skirt, wrist pin bearings connected with said head and said skirt,said skirt having cylinder-engaging portions separated from said had andconstructed to flex inwardly inthe regions of said bearings, and anelement extending circumferentially inside of and attached to each of.said cylinder-engaging portions, said element having a rate ofexpansion lower than that of said portion and adapted to ex said.portion inwardly in said regions thermostatically. I

11. A light metal piston of the character described, comprising, a head,a skirt, wrist pin bearings, connectors between said head and saidskirt, said skirt having cylinder-engaging portions separated from saidhead circumferentially and separated from said connectors terminally,constructed to flex inwardly in the regions of said bearings, andelements having a rate of expansion lower than that of saidcylinder-engaging portions, oneof said elements extendingcircumferentially inside of and cast to each of said portions andterminating therewith, adapted to iiex each said portion inwardly insaid regions thermostatically.

WILLIAM M. VENINER.v PERCY L. BOWSER, JR.

